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The study evaluates the accuracy of measured and estimated crop evapotranspiration fluxes (ETc) on Citrus in a semiarid Mediterranean climate (Sicily, Italy). Specifically, ETc rates derived from in situ techniques [eddy covariance (EC) and sap flow heat pulse velocity (HPV)] and modelling approaches [Food and Agricultural Organization of the United States (FAO) Irrigation and Drainage Paper No. 56 (FAO-56) single and dual crop coefficient (Kc)] were compared under deficit irrigation scenarios. Results of the comparison showed that the single and dual Kc approaches provided similar ETc estimates (292 and 324 mm), even if these approaches overestimated ETc measured by EC (ETEC) (17% and 30% respectively). HPV was able to show transpiration (T) reductions caused by deficit irrigation strategies when compared with T under full irrigation condition (ranging from 70% to 82%). Overall, the assessed methodologies were able to capture ETc trends, but the selection of the most appropriate one will depend on the specific crop and study site characteristics. [ABSTRACT FROM AUTHOR]

In this study, spatially distributed estimates of regional actual evapotranspiration (ET) were obtained using a revised procedure of the so called “triangle method” to parameterize the Priestley–Taylor ϕ coefficient. In the procedure herein proposed, named Time-Domain Triangle Method (TDTM), the triangular feature space was parameterized considering pairs of T s –VI values obtained by exploring, for each pixel, only their temporal dynamics. This new method was developed using time series products provided by MODIS and MSG-SEVIRI sensors. Moreover the proposed procedure does not depend on ancillary data, and it is only based on remotely sensed vegetation indices and day–night time land surface temperature differences. Two different test areas located in Sicily were selected for testing and validating the approach. Satellite ET rates were validated versus directly measured fluxes of mass (ET) obtained by eddy covariance (EC) towers during the observation period 2010–2012. The proposed approach predicts daily ET rates with an acceptable level of accuracy for practical purposes; therefore, the TDTM can be considered as a simple and effective tool to easily estimate, at regional scale, spatial and temporal changes of this key-variable related to water resource management, agriculture, ecology and climate change. [ABSTRACT FROM AUTHOR]

In this paper, based on the analysis of a long-term energy balance monitoring programme, a Bowen ratio-based method (BR) was proposed to resolve the lack of closure of the eddy covariance technique to obtain reliable sensible ( H) and latent heat fluxes (λ E). Evapotranspiration (ET) values determined from the BR method (ET) were compared with the upscaled transpiration data determined by the sap flow heat pulse (HP) technique, evidencing the degree of correspondence between instantaneous transpirational flux at tree level and the micrometeorological measurement of ET at orchard level. Using the BR-corrected λ E fluxes, a crop ET model implementing the Penman-Monteith approach, where the canopy surface resistance was determined from standard microclimatic variables, was applied to determine the crop coefficient values. The performance of the model was evaluated by comparing it with the sap flow HP data. The results of the comparison were satisfactory, and therefore, the proposed methodology may be considered valid for characterizing the ET process for orange orchards grown in a Mediterranean climate. By contrast to reports in the FAO 56 paper, the crop growth coefficient of the orange orchard being studied was not constant throughout the growing season. [ABSTRACT FROM AUTHOR]

Daily evapotranspiration fluxes over the semi-arid Catania Plain area (Eastern Sicily, Italy) were evaluated using remotely sensed data from Landsat Thematic Mapper TM5 images. A one-source parameterization of the surface sensible heat flux exchange using satellite surface temperature has been used. The transfer of sensible and latent heat is described by aerodynamic resistance and surface resistance. Required model inputs are brightness, temperature, fractional vegetation cover or leaf area index, albedo, crop height, roughness lengths, net radiation, air temperature, air humidity and wind speed. The aerodynamic resistance (rah) is formulated on the basis of the Monin-Obukhov surface layer similarity theory and the surface resistance (rs) is evaluated from the energy balance equation. The instantaneous surface flux values were converted into evaporative fraction (EF) over the heterogeneous land surface to derive daily evapotranspiration values. Remote sensing-based assessments of crop water stress (CWSI) were also made in order to identify local irrigation requirements. Evapotranspiration data and crop coefficient values obtained from the approach were compared with: (i) data from the semi-empirical approach "Kc reflectance-based", which integrates satellite data in the visible and NIR regions of the electromagnetic spectrum with ground-based measurements and (ii) surface energy flux measurements collected from a micrometeorological tower located in the experiment area. The expected variability associated with ET flux measurements suggests that the approach-derived surface fluxes were in acceptable agreement with the observations. [ABSTRACT FROM AUTHOR]

Photosynthetically active radiation (PAR) intercepted by orange orchards (Frost Nucellar navel) having different canopy sizes was measured to determine the relationships with crop coefficient (Kco and Kcr) values and crop evapotranspiration (ET) (ETc). Three separate experiments were carried out near Lindsay, Calif. during the months of July and August 2004 to compute the fraction of light PAR intercepted by mature and immature orange orchards. Periodic readings of PAR data were compared with near simultaneous measurements of net radiation Rn (mV), heat transfer through exposed flux plates Fh (mV), and incident total solar radiation Rs (mV). The PAR data were used to calculate canopy light interception and the results were compared with those computed from the Fh and Rs data. The other sensors were studied as possible substitutes for the more expensive PAR light bar. Light interception by the different canopies was related to crop coefficient (Kco and Kcr) values that were determined by micrometeorological measurement of ETc and Penman–Monteith reference evapotranspiration ETo and ETr. [ABSTRACT FROM AUTHOR]

Evapotranspiration (ET) estimation is important for understanding energy exchanges and water cycles. Remote sensing (RS) is the main method used to obtain ET data over large scales. However, owing to surface heterogeneities and different model algorithms, ET estimated from RS products with different spatial resolutions can cause significant uncertainties, whose causes need to be thoroughly analyzed. In this study, the Surface Energy Balance Algorithm for Land (SEBAL) model was selected to explore spatial resolution influences on ET simulations. Three satellite datasets (Landsat Thematic Mapper (TM), Moderate Resolution Imaging Spectroradiometer (MODIS), and Advanced Very High-Resolution Radiometer (AVHRR)) were selected to independently estimate ET in SEBAL model to identify the influence of the spatial scale on ET estimation, and analyze the effects and causes of scale aggregation. Results indicated that: (1) the spatial distributions of ET estimated from the three satellite datasets were similar, with the MODIS-based ET having the largest uncertainty; and (2) aggregating input parameters had limited changes in the net radiation and soil heat fluxes. However, errors in the sensible heat and latent heat fluxes were relatively larger, which were caused by changes in the selection of hot and cold pixels and the NDVI and surface albedo parameters during scale aggregation. The scale errors caused by the model mechanisms were larger than those caused by the land use/cover pattern in the SEBAL model. Overall, this study highlights the impact of spatial scale on ET and provides a better understanding of the scale aggregation effect on ET estimation by RS. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experiment carried out in Southern Italy (Sicily) on the estimation evapotranspiration (ET) in pilot constructed wetlands planted with different species (Chrysopogon zizanioides, Myscanthus x giganteus, Arundo donax, Phragmites australis, and Cyperus papyrus). In the two monitored growing seasons, reference ET0 was calculated with the Penman-Monteith formula, while actual ET and crop coefficients were measured through a water balance and the FAO 56 approach, respectively. The highest average seasonal ET value was observed in Phragmites australis (17.31 mm d−1) followed by Arundo donax (11.23 mm day−1) Chrysopogon zizanioides (8.56 mm day−1), Cyperus papyrus (7.86 mm day−1), and Myscanthus x giganteus (7.35 mm day−1). For all plants, crop coefficient values showed different patterns in relation to growth stages and were strongly correlated with phenological parameters. Myscanthus x giganteus and Arundo donax showed a water use efficiency values significantly higher than those observed for the other tested species. Results of this study may contribute to select appropriate plant species for constructed wetlands located in semi-arid regions, especially when the use of reclaimed water and/or the use of aboveground biomass are planned. [ABSTRACT FROM AUTHOR]

An adjusted satellite-based model was proposed with the aim of improving spatially distributed evapotranspiration (ET) estimates under plant water stress conditions. Remote sensing data and near surface geophysics information, using electrical resistivity tomography (ERT), were used in a revised version of the original dual crop coefficient (Kc) FAO-56 approach. Sentinel 2-A imagery were used to compute vegetation indices (VIs) required for spatially estimating ET. The potentiality of the ERT technique was exploited for tracking the soil wetting distribution patterns during and after irrigation phases. The ERT-derived information helped to accurately estimate the wet exposed fraction (few) and therefore the water evaporated from the soil surface into the dual Kc FAO-56 approach. Results, validated by site-specific ET measurements (ETEC) obtained using the eddy covariance (EC) technique, showed that ERT-adjusted ET estimates (ETERT) were considerably reduced (15%) when compared with the original dual Kc FAO-56 approach (ETFAO), soil evaporation overestimation being the main reason for these discrepancies. Nevertheless, ETFAO and ETERT showed overestimations of 64% and 40% compared to ETEC. This is because both approaches determine ET under standard conditions without water limitation, whereas EC is able to determine ET even under soil water deficit conditions. From the comparison between ETEC and ETERT, the water stress coefficient was experimentally derived, reaching a mean value for the irrigation season of 0.74. The obtained results highlight how new technologies for soil water status monitoring can be incorporated for improving ET estimations, particularly under drip irrigation conditions. [ABSTRACT FROM AUTHOR]